Publications and other reference materials referred to herein, including references cited therein, are incorporated herein by reference in their entirety and are numerically referenced in the following text and are respectively grouped in the appended bibliography which immediately precedes the claims.
Until recently, the conventional methods for pumping active double-clad fibers, as a part of fiber lasers or amplifiers, involved mainly on-axis coupling through a dichroic mirror. On-axis pumping means that both pump and signal (laser) beams share the same optical axis outside of the fiber.
In recent years, there have been many efforts to separate the laser and pump beam directions with various off axis or sidewise pumping configurations. Off-axis (sidewise) pumping means the pump and laser beams are separated and do not share the same optical axis (outside of the fiber) when coupled into the fiber. This has been demonstrated by various methods [1-5]. Some methods are based on altering the pump cladding, such as angle-polishing of the fiber [6], embedded-mirrors [7], V-groove coupling [8] and capillary techniques [9].
However, these methods suffer from low damage threshold due to adhesives, mechanical instability, and require partial damaging of the fibers. Other techniques that have been used include tapering and/or splicing to couple pump fibers to an active fiber [10-13]. Alternatively, there are also techniques in which the pump is reflected into the active fiber by using a dichroic (interference filter) element [14, 15]. Moreover, some techniques used today in commercial fiber lasers and amplifiers are based on the principle of leakage from the pump fibers to the active fiber over a long interaction length [16, 17]. With most of the above methods it is impossible to couple the pump light efficiently into double-clad photonic crystal fibers (PCF) due to their air cladding structure. Additional examples of limitations with the prior art are the need to alter or perform some kind of processing on the fiber or that the end facet of the fiber is not freely usable.
In order to obtain from a fiber laser short laser pulses with high levels of output peak power and good beam quality (in a system with small physical dimensions), it is essential to use active double-clad PCFs. The most significant advantage of a PCF, in this context, is the extremely large core cross section and the high numerical aperture (NA) of the pump cladding, especially with the rod-type PCF. Another key advantage of side pumping is that it allows the use of pump sources with very low spatial coherence, such as arc lamps or high-power diode bars. Also, side pumping makes it easy to combine multiple pump sources. [18]
It is an object of the present invention to provide a new method for free space pumping of active double-clad fiber based lasers and amplifiers.
It is another object of the present invention to provide a new method for free space pumping of active double clad PCF based lasers and amplifiers.
It is another object of the present invention to provide a method that is based on off-axis injection of the pump beam, exploiting the high numerical aperture (NA) of the fiber pump cladding.
It is another object of the present invention to provide a method that can be applied to a variety of different types of active double-clad fibers.
It is another object of the present invention to prevent feedback from the laser into the pump source.
It is another object of the present invitation to provide a method in which the optical elements for the pump and signal beams are completely separated.
It is another object of the present invention to provide a method where there is no requirement for any special treatment to the fiber.
It is another object of the present invention to provide apparatus for carrying out the method of the invention.
Other objects and advantages of the invention will become apparent as the description proceeds.